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FreeBSD-SA-20:08.jail                                       Security Advisory
                                                          The FreeBSD Project

Topic:          Kernel memory disclosure with nested jails

Category:       core
Module:         kern
Announced:      2020-03-19
Credits:        Hans Christian Woithe <chwoithe@yahoo.com>
Affects:        All supported versions of FreeBSD.
Corrected:      2020-03-16 21:12:46 UTC (stable/12, 12.1-STABLE)
                2020-03-19 16:51:33 UTC (releng/12.1, 12.1-RELEASE-p3)
                2020-03-16 21:12:32 UTC (stable/11, 11.3-STABLE)
                2020-03-19 16:51:33 UTC (releng/11.3, 11.3-RELEASE-p7)
CVE Name:       CVE-2020-7453

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.

I.   Background

The jail_set(2) system call allows a system administrator to lock up a
process and all its descendants inside a closed environment with very
limited ability to affect the system outside that environment, even
for processes with superuser privileges.

The jail_get(2) system call allows a system administrator to read the
configuration of running jails.

II.  Problem Description

A missing NUL-termination check for the jail_set(2) configration option
"osrelease" may return more bytes when reading the jail configuration
back with jail_get(2) than were originally set.

III. Impact

For jails with a non-default setting of children.max > 0 ("nested jails")
a superuser inside a jail can create a jail and may be able to read and
take advantage of exposed kernel memory.

IV.  Workaround

No workaround is available.  Systems not altering the default settings of
the jail configuration option children.max=0 are not affected as a root on
the base system has access to kernel memory by other means and a super
user inside a jail cannot create further jails.

V.   Solution

Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date,
and reboot.

Perform one of the following:

1) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for a security update"

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/SA-20:08/kern_jail.patch
# fetch https://security.FreeBSD.org/patches/SA-20:08/kern_jail.patch.asc
# gpg --verify kern_jail.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/12/                                                        r359021
releng/12.1/                                                      r359142
stable/11/                                                        r359020
releng/11.3/                                                      r359142
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-7453>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-20:08.jail.asc>
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